Transit authorities planning to transition from conventional to CBTC signaling must treat the depot and mainline as a single entity; otherwise the boundary becomes a barrier for launching trains into service. The barrier results from CBTC and conventional signalling speaking different languages; a simplified interface will lose something in translation, preventing a seamless handover of a train from depot to mainline.

Transit agencies planning to deploy a CBTC solution must be mindful that a CBTC solution is effective only when it has control over all aspects that affect mainline operations. The time it takes to launch trains from the depot is also a factor because it compromises the throughput on the mainline. Non-CBTC actors, such as a conventionally signalled depot, hinder a CBTC solution’s ability to control the flow of trains on the mainline, reducing the advantages CBTC was meant to introduce.

Implementing a CBTC solution on the mainline and leaving the depot conventionally signalled is a mistake.

The positional accuracy of a GPS depends on the environment it’s operating in; if six satellites are in the sky on a clear sunny day in the countryside, the positional accuracy will be high. But if there is a thunderstorm in the area with only 3 satellites in the middle of a metropolis, the positional accuracy diminishes. Pinpoint accuracy requires ideal conditions.

Similarly, a CBTC train’s ability to report an accurate position depends on the design of the train and the track it’s running on. Train parameters and track characteristics must be understood and controlled to reduce positional uncertainty; otherwise the train cannot be tracked nor can it be protected.

During my teenage years, my driving instructor taught me the three second rule; the minimum separation between my car and the car in front had to be three seconds. He drilled this rule into my head, but a rule for cars does not translate well when driving a fifty ton train at 60 km/h around a blind curve. Something a little more sophisticated is required.

The equivalent guideline in the conventional signalling world is the “one block separation” rule; the distance separating two trains must be one block. This 150 year old practice ensures that if a train is tripped there is enough distance to stop the train.

Tracking trains using track circuits has been the conventional wisdom for the past 150 years. No single invention in the history of rail has contributed more towards safety then the track circuit. This simple invention is the foundation for block signalling and the primary method of tracking trains through a mass transit system.

However, over the past 30 years CBTC technologies have rendered this foundational component obsolete. Therefore, how does a CBTC system track trains without track circuits?